It is now 15 years since radars were first used intensively to probe the atmosphere in clear-air conditions. The first experiments were concerned with the nature of the targets and accounting for the intensity of the measured echoes. It was found that inhomogeneities in the refractive index field were responsible for a large proportion of clear-air returns. Such returns were detected in characteristic patterns in association with sheared statically stable layers, convective thermals, air mass boundaries, and a variety of wave structures. More sensitive Doppler radars have been developed in recent years which are able to obtain usable signals from all heights in the troposphere. Such radars, as well as providing additional information about the clear-air patterns observed previously, enable continuous measurements of wind profiles to be made. This paper describes the phenomena which have been observed and the measurements which have been made with radars under clear-air conditions. A summary is given of the types of targets observed, and the kinds of radars used in clear-air studies are briefly described.